The eutrophication of lakes, rivers and other water resources is receiving worldwide attention. The presence in the environment of nutrients, such as phosphate and nitrogen is one of the primary causes of eutrophication. These nutrients promote unwanted growth of algae and other aquatic plants.
The eutrophication of our lakes and rivers has led to increased demands for nutrient control in wastewater treatment plants. Governmental agencies have enacted increasingly stringent regulations controlling the amount of nutrients which can be discharged into receiving waters. Since conventional treatment processes remove only small amounts of nitrogen and phosphate, wastewater treatment plants will be required to change or modify their processes to meet these increasingly stringent regulations. Unfortunately, the technology to achieve the required removal efficiencies is lagging behind regulatory requirements.
One approach for accomplishing nutrient removal is biological treatment in a modified activated sludge system without chemical addition. Numerous biological nutrient removal processes have been developed. These biological nutrient removal processes typically use a single sludge configuration in which the organic matter of the influent is used as the carbon and energy source for nitrogen and phosphate removal. This allows for lower operating cost in comparison to multiple sludge systems and other physical-chemical systems.
In such biological systems, removal of nitrogen from wastewater is conventionally done by first nitrifying (converting ammonia to nitrite and nitrate, NO.sub.x) and then denitrifying (reducing NO.sub.x to N.sub.2) the wastewater. The process of nitrification is carried out in an aerobic environment by autotrophic organisms which derive energy for growth by oxidizing ammonia nitrogen values to NO.sub.x. The energy produced is then utilized to produce protein material from inorganic components in the wastewater, such as H.sub.2 O, CO.sub.2 and NH.sub.3. Denitrification is typically carried out in an oxygen-less environment by heterotrophic organisms which utilize NO.sub.x in the absence of oxygen as an electron acceptor for oxidation of sorbed organic compounds.
One biological nutrient removal process which is commonly used is known as the Bardenpho Process. The Bardenpho Process consists of an initial anaerobic contact zone followed by four alternating stages of anoxic and aerobic conditions. In the anaerobic zone, all of the raw wastewater is mixed with the return sludge. The anaerobic conditions in the initial contact zone is necessary to effect phosphate removal. The first anoxic zone follows the anaerobic zone. Nitrates and nitrites (NO.sub.x) are supplied to the anoxic zone by recycling nitrified mixed liquor from the following aerobic zone. The organic material in the raw wastewater is used as a carbon source by the denitrifying bacteria in the first anoxic zone to reduce NO.sub.x to elemental nitrogen or nitrous oxide. The first aerobic (oxic) zone is followed by a second anoxic zone where any remaining NO.sub.x in the mixed liquor is reduced by the endogenous respiration of the activated sludge. The final stage is aerobic where the mixed liquor is reaerated before reaching the final clarifier. The dissolved oxygen of the wastewater effluent is increased to prevent further denitrification in the clarifier and to prevent the release of phosphates to the liquid in the clarifier.
The Bardenpho Process is capable of achieving a high percentage of nitrogen compound removal as well as phosphate removal. However, the Bardenpho Process requires substantially larger tank volumes than conventional activated sludge systems which means higher capital outlays. Additionally, the Bardenpho System relies on endogenous respiration in the second anoxic reactor which is a relatively slow process. Thus, its use is limited to small plants.
Another biological nutrient removal process which is frequently used is known in the industry as the A.sup.2 O Process. The A.sup.2 O process consists of three treatment zones--anaerobic, anoxic and aerobic. The wastewater and return sludge are mixed in the first treatment zone which is maintained under anaerobic conditions to promote phosphate removal. The anaerobic zone is followed by an anoxic zone. The third treatment zone is an aerobic zone where nitrification of the mixed liquor is achieved. The nitrified mixed liquor is recycled back to the anoxic zone where the nitrate and nitrite is reduced to elemental nitrogen and/or nitrous oxide by denitrifying organisms. The A.sup.2 O system has a high rate of nitrogen removal and requires total tank volume comparable to that of conventional activated sludge systems. Thus, the A.sup.2 O system is a cost effective system for nutrient removal. However, the A.sup.2 O system requires a relatively high mixed liquor recycle rate in order to achieve high nitrogen removal efficiency.
Accordingly, there is a need for a biological nutrient removal process which accomplishes high nitrogen removal efficiencies, which is cost effective, and which minimizes capital outlays required to retrofit conventional activated sludge systems.